Double metal cyanide complex catalysts are known to be extremely useful and active catalysts for the ring-opening polymerization of heterocyclic monomers such as epoxides. In particular, catalysts of this type have been employed to prepare polyether polyols derived from propylene oxide as described, for example, in U.S. Pat. Nos. 3,404,109, 3,829,505, 3,900,518, 3,941,849, 4,355,188, 5,032,671, and 4,472,560. Double metal cyanide complex catalysts are generally recognized as superior to the caustic catalysts traditionally used to produce polyether polyols for utilization in polyurethane foams, coatings, adhesives, sealants, and elastomers due to the reduced levels of unsaturation and higher functionality of the polyols obtained using such catalysts, as described in U.S. Pat. Nos. 4,239,879, 4,242,490 and 4,985,491.
The double metal cyanide complex catalysts are generally prepared by reacting a metal salt such as zinc chloride with an alkali metal hexacyanometallate such as potassium hexacyanocobaltate in aqueous solution. The resulting water-insoluble double metal cyanide complex catalyst which precipitates from solution must thereafter be recovered from the aqueous reaction medium, washed to remove undesired by-products and impurities, and dried in order to obtain the catalyst in a form suitable for use in a polymerization process. The isolation of the dry, active double metal cyanide complex catalyst is complicated by the extremely fine particle size of the precipitated catalyst, however. Direct filtration by conventional means is not practicable, particularly when the catalyst slurry is treated with an activating organic ligand such as dimethoxyethane. The addition of ligand often greatly increases the viscosity of the slurry such that an intractable gel or paste is formed even with a relatively large volume of liquid present. Centrifugation, dialysis, or conversion to a filterable metal hydroxide species are methods suggested in the literature for accomplishing the isolation. Various methods are described in U.S. Pat. Nos. 3,427,256, 3,941,849, 4,472,560, and 4,477,589. Alternatively, filter-aid particles may be added to the aqueous reaction mixture to facilitate the filtration as described in U.S. Pat. No. 4,843,054. These prior art methods all suffer from certain disadvantages such as the use of sophisticated, expensive, or low capacity equipment, the need for subsequent reaction steps to convert the recovered precipitate to a catalytically active form, or the dilution of the active catalyst with an inert filter-aid. The present invention provides a convenient and effective method of preparing an active double metal cyanide complex catalyst which can be readily isolated by conventional and straight-forward filtration techniques.